Hydraulic fluid controls can be found in a variety of automotive applications such as automatic speed change transmissions as well as others. In these applications, it is often desirable to control the pressure of the hydraulic fluid. For example, in a common configuration, an electronic transmission control unit (TCU) for an automatic speed change transmission is configured to generate electrical signals that control solenoids, which in turn results in the control of fluid flow as well as the pressure in various hydraulic fluid lines. The pressure in a hydraulic fluid line can be used to control various other elements including, for example, a hydraulically-actuated clutch for the engagement of individual gears. By engaging various combinations of gears (e.g., planetary gears in a planetary gear transmission), an automatic transmission accomplishes the same task as the shifting of gears in a manual transmission.
In the automatic transmission example, it is also known to provide a solenoid-operated fluid valve, or as often referred to in this context as simply a solenoid (or sometimes actuator). For example, the solenoid can be used to control the hydraulic fluid pressure to apply and/or release the clutch. In a linear solenoid, the amount of fluid at a controlled pressure can be varied by changing a solenoid control current. To achieve control of a system including a linear solenoid, it is known to employ software responsive to various inputs to control the current applied to the solenoid. Various configurations including a linear solenoid are known, including a so-called 2-stage arrangement where a linear solenoid is used to provide a pilot pressure, which is in turn used to control a spool valve or the like, also resulting in the hydraulic fluid pressure at the spool valve outlet being controlled.
It is further known to control the operation of these solenoid-operated valves in either an open-loop control system or in a closed-loop control system. In a conventional open loop control system, it is conventional to employ extensive characterization strategies to characterize the transfer characteristic of the solenoid to the greatest extent possible in order to deliver accurate pressure control. That is, much effort is done in advance of the actual use of the solenoid to develop a highly accurate pressure-current (P-I) data table or map. In a closed loop control system, however, extensive characterization is not normally required. This is because typically encountered levels of variation (i.e., error in the actual output pressure relative to a desired output) can be reduced to manageable levels through feedback. Sometimes, however, the encountered variation can exceed the correction capabilities of the closed loop feedback control system, causing instability and/or poor performance. There are many causes. Occasionally there are significant part-to-part variations in the transfer characteristic of the solenoid, for example, as a result of manufacturing. Additionally, the actual transfer characteristic can vary based on factors that apply after manufacture/deployment, such as changes with temperature, fluid quality, supply pressure, the age of the solenoid as well as its wear/usage, among other factors.
Approaches taken in the art have focused on either (i) refining the post-manufacture, pre-deployment accuracy of a solenoid's transfer characteristic (e.g., P-I table), which is then used in generating a feed forward input control signal, or in (ii) refining the closed-loop feedback strategy itself. For example, as to the former approach, it is known to provide a one-time post-manufacture calibration of a solenoid's P-I map, as seen by reference to U.S. Pat. No. 6,751,542 entitled “CORRECTIVE CONTROL SYSTEM AND METHOD FOR LIQUID PRESSURE CONTROL APPARATUS IN AUTOMATIC TRANSMISSION” to Ishii et al. However, the refined P-I map is static once it has been calculated. Accordingly, Ishii et al. do not provide a mechanism to address variations that occur during real-time usage of the solenoid and/or over time. Moreover, the approach disclosed in Ishii et al. is too complicated for real-time usage, as it relies on complex curve fitting algorithms (e.g., Sum of Squares), which involves iteration, significant computing resources and memory, as well as the availability of a special test mode needed to exercise the complete range of the solenoid's transfer characteristic to obtain a full and complete data set. Most real-time systems can not accommodate the main controller being programmed to discontinue its control function for the purpose of executing a test program to implement the kind of calibration taught in Ishii et al. Accordingly, Ishii et al. do not effectively address the problems described above.
Accordingly, there remains a need for a system for operating a solenoid-operated valve in a pressure control system that minimizes or eliminates one or more of the shortcomings described above.